The following devices are well known as carburetors that supply fuel to two-cycle or four-cycle general purpose engines: fixed-venturi carburetors that comprise a butterfly-type throttle valve and two fuel systems composed of a main system and a speed-reducing system as shown in Japanese Patent Application Laid-open No. 55-69748, and other publications; variable-venturi carburetors that comprise a cylindrical sliding throttle valve and a single fuel system in which the fuel delivery quantity is made variable by way of a metering needle as shown in Japanese Utility Model Publication No. 49-17682, and other publications; and carburetors that comprise a cylindrical rotating throttle valve and a single fuel system in which the fuel delivery quantity is made variable by way of a metering needle as shown in Japanese Patent Application Laid-open No. 58-101253, and other publications.
Because a carburetor comprising a sliding throttle valve or rotating throttle valve is configured with a single fuel system, it has advantages in that the fuel passage structure is simple in comparison with one comprising a main system and a speed-reducing system, and the fuel consumption from low speeds to high speeds is smooth. In common practice, carburetors comprising a single fuel system and a sliding-type or rotating-type throttle valve have a diaphragm-type constant fuel chamber.
When an engine to which fuel is supplied by the carburetor described above is operated, and particularly when the constant fuel chamber is a diaphragm type, the diaphragm may malfunction, the fuel may leak, and other problems may be brought about by engine vibration and the discharge of residual air from the constant fuel chamber, in addition to atmospheric temperature, pressure, fuel temperature, and other external conditions, and because of these factors, marked fluctuations in rotational speed of the engine cannot be avoided even at rated load operation. Marked fluctuations in the rotational speed of the engine result are an impediment in terms of achieving a smooth operation during outdoor work when using a lawn mower or the like, and also result in an increase of toxic substances in the exhaust.
Conversely, a lean mixture in a narrow air-fuel ratio range is required in order to operate an engine with good stability while reducing fuel consumption. Any inconsistencies in the fuel delivery quantity caused by variation in component precision (inherent in the carburetor itself) during manufacture, in addition to discharge of residual air, fuel leaking, and other above-described phenomena that occur during service, make it difficult to maintain a lean mixture in a narrow air-fuel ratio range.